浙江安吉铅锌多金属矿区细粒花岗岩的岩石化学、年代学及成矿意义探讨

文章在详细的野外地质调查、岩石学、岩石化学和锆石U-Pb定年结果基础上,对浙江安吉矿区细粒花岗岩的岩浆起源、侵位时代与成矿的关系及找矿指示意义等进行了较为深入的探讨.细粒花岗岩侵位时代日于矿区的黑云母二长花岗岩、正长花岗岩和花岗闪长岩,侵位时代约在134 Ma左右,为中国东部东侏罗世一早白垩世岩石圈活化及大规模成岩成矿作用的产物.岩石化学研究结果表明,细粒花岗岩属高钾钙碱性钾玄岩、过铝质岩石系列,轻稀士元素相对富集,重稀土元素相对亏损,富集大离子亲石元素Rb、Th、U、K、轻稀土元素和Pb,明显亏损高场强元素Nb、Ta、Ti和P,显示较高的分异程度.据根锆饱和温度,计算其母岩浆结晶温度约在834.6 ～ 870.6℃.结合矿区已知的矿化和蚀变与细粒花岗岩的空间关系、细粒花岗岩岩石化学特征以及细粒花岗岩与矿石铅同位素研究结果,笔者认为,安吉铅锌多金属矿区铅锌钼矿化与细粒花岗岩具有密切的成因联系,细粒花岗岩与寒武系灰岩接触部位控制了矽卡岩型铅锌矿化的产出,这一成果对该区的进一步找矿勘查具有重要的指导意义.

Petrochemistry, chronology and ore-forming geological significance of fine crystalline granite in Anji polymetallic deposit of Zhejiang Province

In the Anji ore district located in northern Qinzhou-Hangzhou metallogenic belt,there exists the Wushanguan complex body in which the rock units have complicated intrusion sequences and are related to alteration and mineralization.In order to study the relationship between magmatic activity and mineralization,the authors analyzed the major and trace elements concentration and the characteristics of petrogeochemistry and petrogenesis of Wushanguan complex body based on the results of field work and petrographic studies.The major and trace elements concentrations were tested by XRF and ICP-MS.Wushanguan complex body is mainly composed of biotite-monzonitic granite,granodiorite and syenogranite.The biotite-monzonitic granite is characterized by alkali-rich and potassium-high nature,and belongs to quasi-aluminous rock and shoshonite series.The granodiorite is characterized by high potassium and belongs to quasi-aluminous rock and high-K calc-alkaline series.Syenogranite is similar to A-type granite characterized by high SiO2,low CaO,low MgO,high FeO*/MgO ratio,relatively rich alkali and high potassium,and belongs to metaluminous-weakly peraluminous rock and shoshonite series.The total concentrations of the rare earth elements of the complex range from 123.5×10-6 to 216.47×10-6.The rare earth elements distribution patterns of the complex are all of LREE-enriched type 〔(La/Yb)N=7.47～23.93〕 with different negative Eu anomalies(Eu/Eu*=0.38～0.86).In the primitive mantle normalized multi-element variation diagram,large-ion lithophile elements,such as Rb,Th and K,are abundant with obvious negative anomalies of Ba and Sr,and the rocks are all relatively depleted in some high field strength elements such as Nb,Ti and P.Among the ore-forming elements,the element values of Mo and W are higher than their Clarke values obviously,that is to say,the complex has the potential of becoming a mineralization source.According to the characteristics of major elements and trace elements,especially the discrimination diagrams,biotite-monzonitic granite is separately fractionated I-type granites and the granodiorite belongs to I-type granite.Syenogranite has some characteristics of highly fractionated A-type granite,but there are several reasons suggesting that it does not belong to A-type granite:firstly,A-type granites generally do not exhibit evidence of being strongly differentiated;secondly,on the whole,all the complex members have a similar age within the experimental error as well as have an affinity to each other in geochemistry,so that all the rock units should be formed in the same tectonic setting.Considering that biotite-monzonitic granite and granodiorite belong to I-type granite,it is more reasonable to assign moyite to highly fractionated I-type granite.The La/Sm-La diagram shows that the complex body was formed by equilibrium fusion.Light rare earth elements enrichment and Ta-Nb-Ti,Eu,Sr and Ba negative anomalies probably resulted from hornblende group,when rutile and plagioclase stayed in residue in source region.